A gear device has been used in various technical fields such as industrial robots and machine tools (see Japanese Patent Application Publication No. 2016-109264 (the '264 Publication)). Such a gear device is provided with an outer cylinder, a carrier, and a main bearing. The outer cylinder encircles the carrier. The main bearing is disposed in an annular space formed between the outer cylinder and the carrier and defines a rotation axis. The carrier rotates relative to the outer cylinder about the rotation axis.
The main bearing described in the above '264 Publication defines an acting direction of a load applied to the gear device. The main bearing described in the above '264 Publication defines a load action line inclined with respect to the above-mentioned rotation axis.